✨ Give fc.statistics a return value

[Copilot]

The fc.statistics function now returns classification data instead of only logging it, enabling programmatic validation of arbitrary distributions.

Changes

• New StatisticsReport type with classes (label→count map), numRuns, and numSkipped fields
• Updated statistics function to return StatisticsReport while preserving existing logging behavior
• Exported type from main package for TypeScript consumers

Usage

// Property test your arbitraries by asserting on distributions
const report = fc.statistics(
  fc.string(),
  (v) => `${v.length} characters`,
  { numRuns: 100_000 }
);

// Assert on coverage
assert(report.classes['1 characters'] > 0);
assert(Object.keys(report.classes).length > 1); // Has variety

// Or use in property tests
fc.assert(fc.property(fc.integer(), (seed) => {
  const results = fc.statistics(
    myCustomArbitrary,
    classifier,
    { numRuns: 1000, seed, logger: () => {} }
  );
  return results.classes['edge_case'] > 0;
}));

The numSkipped field tracks values where the classifier returned an empty array. Multiple classifications per value are supported—numSkipped counts unique values, not total classifications.

Original prompt

fix #4342

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